1. Field of the Invention
This invention relates generally to systems for locating and tracing buried objects and more particularly to a system for simultaneously detecting and tracing several buried objects by means of various combinations of active and passive blind signal detection.
2. Description of the Related Art
There are many situations where is it desirable to locate buried utilities such as pipes and cables. For example, prior to starting any new construction that involves excavation, it is important to locate existing underground utilities such as underground power lines, gas lines, phone lines, fiber optic cable conduits, CATV cables, sprinkler control wiring, water pipes, sewer pipes, etc., collectively and individually herein denominated “buried objects.” As used herein, the term “buried objects” includes objects located inside walls, between floors in multi-story buildings or cast into concrete slabs, for example, as well as objects disposed below the surface of the ground. If excavation equipment such as a backhoe hits a high voltage line or a gas line, serious injury and property damage may result. Unintended severing of water mains and sewer lines generally leads to messy and expensive cleanup efforts. The unintended destruction of power and data cables may seriously disrupt the comfort and convenience of residents and bring huge financial costs to business.
Accordingly, many solutions to the buried object locating problem have been proposed by practitioners in the art. For example, buried objects may be “passively” located by sensing any electromagnetic signals emitted by the object. Some cables, such as power lines and some communication lines, for example, are already energized and therefore characterized by the emission of an electromagnetic signal, which may be embodied as an elongated electromagnetic field. Other buried objects, such as conductive lines and pipes, may be “actively” located by first applying an external electromagnetic signal to the object and thereby energizing the object to characterize it by the emission of a portion of the external electromagnetic signal. For example, an external electrical signal source having a frequency in a range of approximately 4 Hz to 500 kHz has a well-known utility for energizing conductive objects to permit their passive location when provided with means for external conductive coupling to the object sought. The active/passive location of buried long conductors is commonly denominated “line tracing.”
A sonde (also commonly denominated a transmitter, beacon or duct probe) is another embodiment of an external electrical signal source that has a well-known utility for marking nonconductive buried objects to permit their passive location. A sonde typically includes a coil of wire wrapped around a ferromagnetic core that is packaged for insertion into buried nonconductive conduits such as plastic utility runways or concrete water pipes, for example. The coil may be energized with a standard electrical source at a desired frequency, typically in a range of approximately 4 Hz to 500 kHz. The sonde may be attached to a push cable or line or it may be self-contained so that it can be flushed into the conduit with water. A sonde generates a more complex and localized electromagnetic field than that produced by an energized line and may be localized to a single point by exploiting these complexities. A typical low frequency sonde that is useful for single-point localizing does not strongly couple to other objects and thereby avoids the production of complex and unpredictable interfering fields. The term “buried objects” as used herein also includes external buried sondes and passive markers such as the passive marker described by Galloway in U.S. Pat. No. 5,699,048 or Bolson in U.S. Pat. No. 4,712,094, for example.
When locating buried objects prior to excavation, it is further desirable to determine the approximate depth of the objects. This is generally attempted by measuring the characterizing emission field strength at two locations and analyzing the differences.
There are many instances where the land that is to be excavated may be traversed or crisscrossed by several different utilities such as an AC cable, a water line, a gas line, a sewer pipe and a communications line. It is highly desirable to be able to determine their paths and their depths all at the same time. Some transmitters known in the art can produce several different signals at different frequencies for application to the same underground object or even to different underground objects, but a problem with these systems arises when several pipes are located in the same area and the location of all pipes is desired. Signals transmitted by several pipes can interfere and complicate the detection process.
Many practitioners propose various solutions to this multiple signal detection and separation problem. For example, in U.S. Pat. No. 5,264,795, Rider discloses a locating system that includes an encoded digital or analog signal in the transmitter carrier to facilitate identification of a conduit during its active location. Rider neither considers nor suggests solutions to the related passive location problem. More recently, in U.S. Pat. No. 6,211,807, Wilkison discloses a system that actively locates a buried electrically-conductive object characterized by the emission of a spread-spectrum signal impressed on the object. While Wilkison's system is advantageous for the active location of a particular buried object in the presence of “noise” that includes other signal emissions from other buried objects because of the improved signal-to-noise ratio (SNR) and other advantages of spread-spectrum signal processing, these advantages are not applicable to the related passive location problem.
Another problem with buried object locators known in the art is the signal phase relationships from a 3-axis sensor array, which are ambiguous and cannot alone facilitate assembly of the field vector components in the correct quadrant. In the United Kingdom, for example, where passive detection of broadband emissions in the 15–24 kHz band is typically used to find buried wires, available broadband detection methods yield no clearly-defined phase, leaving unresolved ambiguities in the resulting field vector.
Another well-known portable locator problem is the convolution of any locator system operator motion with the received emission signal. In operation, a portable locator system is typically swung side to side or about in a large arc to obtain an initial estimate of the direction to a buried object such as a utility line. Even when tracing a buried utility line following its detection, small path deviations may cause signal phase reversals at the sensors whose axes are aligned substantially perpendicular to the local field vector. This problem arises in some sensor coil embodiments where sensor signal sensitivity is generally proportional to the sine of the angle between the sensor coil axis and the local field vector. When the sensor coil axis is substantially aligned with the local field, the near-zero value for the sine of that angle fluctuates rapidly with the angle so that minute angular changes arising from locator system motion can cause large changes in sensor signal amplitude and phase. Practitioners in the art have proposed no solutions for this particular disadvantageous effect of operator motion.
Accordingly, there is still a clearly-felt need in the art for an improved method for the active and passive detection of buried objects in a crowded or noisy environment. Such a system must provide for the simultaneous detection and identification of either a passively-emitting buried object such as an energized power cable or an actively-energized buried object such as a conductive pipe energized by means of an external transmitter signal or a non-conductive conduit occupied by an energized sonde, or all three such objects simultaneously, for example. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.